1. Field of the Invention
The present invention relates to low pass electronic filters using high value resistors and capacitors suitable for fabricating on a silicon IC (Integrated Circuit), and more particularly to a low pass filter that attenuates the audio frequency range, but is efficient in its use of chip area.
2. Background Information
Audio frequency electronic components typically require high value capacitors that are unsuited for placing in silicon ICs since the chip area needed is large. Large resistors may be more easily obtained in silicon, but large value capacitors are very inefficient with respect to chip area. For example a 0.1 microFarad capacitor may use as much as 4×107 square microns or (40 square millimeters) of chip area. The audio range is defined herein to extend from about 20 Hz (Hertz) to 10K Hz.
In some audio systems, digital (logic) circuits are integrated onto the same chip. One issue with these audio systems is that the audio signals might find their way into the logic circuitry where errors may result. For example as in FIG. 1, if a digital circuit component, say a N-type CMOS 2 finds an audio signal riding on a typical DC voltage of +1.3V at its gate A, the low going portion of the audio signal may reach the gate threshold and erroneously turn CMOS off. However, if A was at a DC level of +1.3 V, the CMOS 2 threshold was at 0.7V, and the audio signal had a negative peak voltage of say 0.7 V, point A would drop (1.3V-0.7V) to +0.6V which would turn off the CMOS 2. The result is that the output B would go high when it should not do so. The present invention is directed to relieving such a problem.
As mentioned above, fabricating audio frequency range components, large resistors and large capacitors, on silicon chips is a problem that others addressed. U.S. Pat. No. 6,897,528 B2, to Said Al-Sarawi (hereinafter '528 patent) is one example. This patent discloses a very high value floating CMOS resistor in the gigaOhms range that can be combined with small capacitors to form components useful in the audio frequency range. The '528 patent's high value resistors allows the capacitors to be small enough to make audio IC circuitry practical.
The '528 patent, however, uses a dual balanced combination of CMOS devices to achieve the “floating” characteristic. The “floating” allows the resistor to be fairly unrestricted with respect to ground or power supplies, however, the '528 patent invention requires a constant DC power drain and more chip area (due to its dual nature) as compared to the present invention.